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Searching for Schrödinger’s cat – How to turn photon quantum entanglement into a choreography

It was a sunny day in August 2013 when my friend Margherita and I decided that my PhD thesis on ‘Generation and manipulation of multiphoton quantum states of light’ could be an interesting dance subject.

Our aim was to participate in the ‘Dance your PhD’ contest. The summer heat and the clear Liguria Sea in front of us provided the right inspiration. I began to tell Margherita what my thesis was about. For three years, I had used light — specifically, the fundamental particle of the electromagnetic field, the photon — in order to explore the transition from the microscopic to the macroscopic world. To enlighten this micro-macro transition, I started with a funny story: the Schröedinger’s cat paradox.

Schoredinger’s cat paradox. Credits: Andrea Parisi

The paradox revolves around a box containing a cat, a radioactive nucleus and a bottle of poison. A decayed nucleus would hit the bottle so killing the cat; or, on the contrary, if the nucleus is not-decayed the cat is safe. Schematically, a state of the nucleus decayed or non-decayed in the microscopic quantum world corresponds to a state of the cat dead or alive in the macroscopic classical world. But quantum mechanics tells us that the nucleus can also be in a superimposed state, decayed and not-decayed simultaneously. Correspondingly, the cat should be in a superposition state dead and alive, which is impossible in the classical world. The fate of the cat remains unknown until we open the box. This paradox illustrates the difference between the behavior of microscopic and macroscopic objects.

So, how do we connect the microscopic world and the macroscopic one? Where is the border between the quantum laws, which describe the micro, and the classical laws, which depict the macro world? On the one hand, these fundamental issues concern the basics of the natural behavior. On the other hand, they raise intriguing questions also from a technological point of view. Nowadays a big effort is devoted to the application of quantum technologies to several purposes, such as the realization of the so called “quantum computer”, for which keeping the quantum coherence in increasing size systems is a key ingredient.

The goal of my PhD thesis was to investigate the micro-macro transition by using light. In other words, we wanted to reproduce the “Schrödinger’s cat” state in the context of quantum optics. To this aim, the first thing we did was to generate, in a laboratory, peculiar quantum states which have no analogue in the classical world: the so-called “entangled” photons. The main feature of entangled states relies on the fact that, by probing one of the two photons, we can infer the properties of the other one. In principle, if we send one of the two photons to Mars, their properties remain correlated.

Then, in order to switch from the microscopic world of a single photon to the macroscopic world, we amplified one of the two photons through a non-linear optical process, obtaining thousands of clones at the exit of the amplifier. By measuring in an appropriate way the multiphoton state, we could finally infer the joint properties among the initial single photon and the multiphoton state belonging to the amplification of its entangled twin. Is it still entangled? Could we observe quantum properties also in the micro-macro state?

Let us postpone the answer to the end of the dancing video and come back to the original plan: to turn my PhD thesis into a dance…

Indeed, two coincidences finally convinced us that we had to perform the thesis. Firstly, my twin sister Silvia and I accidentally discovered that we share the same birthday as Schrödinger (August 12th). Secondly, my twin sister would be perfectly suited in the role of my entangled photon.

Silvia and I had classic dance lessons when we were young, so we contacted our teacher and friend, Evelin, a choreographer. We also needed a director, and thus promptly involved Serena, Margherita’s friend and colleague. On a bright Saturday in September at nine in the morning, laboratory mates and some friends, wearing a pair of jeans and a white T-shirt, convened for shooting in front of Sapienza University of Rome. The whiteness of the location, scattering the light around widely, fitted well with our purpose. And the dance started.

Silvia and I enter the stage performing the twin-entangled photons, over the enlightening touch of Bach notes. By dancing as if we are mirroring each other, we show correlated properties, the opposite polarizations.

Then a special crystal amplifies me, generating a bunch of clones that now represent the macroscopic world. The percussion rhythm of a piece by Giovanni Sollima begins to play as the experiment continues.

Is the stand-alone, ‘microscopic’ Silvia still entangled with my new enlarged family? Unfortunately, we cannot get the answer immediately after a trivial measurement. The fact that among several copies of mine, some Silvia-like photons also come out from the crystal, requires the presence of a director, or a filter, whose role is to identify the correct multi-photon state that is actually entangled with the microstate. People-photons create oscillation-based and rotation-based movements representing two possible ways of measuring polarization. After each performance, the filter counts the number of photons moving like me, and those moving like Silvia. He approves the state only when photons are sufficiently unbalanced, that is to say, more moving like me than like Silvia. Otherwise, he rejects the state.

How happy the macro state photons are when they can run towards their micro state entangled counterparts! The two successful measurements suggest the entangled nature between the microscopic-Silvia state and my macroscopic state is one, answering positively our two preliminary questions.

Physicists are intensely searching for answers to the many remaining questions, as shown in the video by the homage to Schrödinger’s cat.

Margherita Cappelletto (left) is technologist at the National Research Council of Italy. She graduated in Astronomy and Astrophysics at Sapienza University of Rome, and received a Master in Science Communication from the International School for Advanced Studies of Trieste. Since then, she works on science dissemination, writing articles, delivering multimedia products and designing performances.

Chiara Vitelli (right) graduated in Physics at Sapienza University of Rome in 2007. She earned a PhD in Physics at the University of Roma Tre in 2011, developing an experimental thesis in Quantum Optics and Quantum Information. Since then she works in the Quantum Optics Group of Sapienza University and at the Italian Institute of Technology in Rome.